Question: I was talking with some friends who ride Harley-Davidsons about what type of motorcycle the Fonz rode on Happy Days, but nobody could come up with an answer. Did we ever know? Paulette Cohn
Televisionary: You just had to send me a toughie that will most likely get me bombed with mail telling me what an idiot I am, huh, Paulette? (I mean, more than the usual amount.)
The answer to your question is that we sort of know, though I'm sure it won't be to the satisfaction of die-hard Harley aficionados, so let me say up front that I know this answer is a little vague. In my book, it's better to commit that sin than attempt to nail the details and get it wrong. Unless, of course, I can convince everyone I'm dead sure of what I'm talking about, in which case I'll let the false details fly. (Warning: The previous comment was inserted to see if The Televisionary's boss is paying attention.)
Anyway, it seems Arthur "Fonzie" Fonzarelli (Henry Winkler) rode at least two different bikes on the show, which enjoyed a long run on ABC from January of 1974 to July of '84. The initial model, ridden in the earlier episodes, is the subject of some dispute among bike buffs. The best I could find is that it was most likely a Harley Sportster.
In an online chat, Winkler himself merely refers to the early bike as a "hog," but adds that it was so big all he could do was lean against it. He identifies the bike ridden in the later episodes as a Harley Triumph and at least one of the other sources I stumbled upon took it further, guessing that it was a "pre-unit 500 Triumph twin."
This is me bearing my inadequacies here, folks I can't pretend to be any more authoritative than that when it comes to cycles.